The present invention relates to a fuel cell system and a controlling method of the fuel cell system in which a purged fuel gas (to be referred to as “hydrogen” hereafter) is discharged after it is diluted and its gas concentration becomes below a predetermined level.
In recent years intensive research and development on the fuel cell to be utilized for automobile vehicles have been under way. In a fuel cell which is a principal part of the fuel system, electrical energy is generated from the electro-chemical reaction between hydrogen supplied to an anode and oxidizing gas (to be referred to as “air” hereinafter) supplied to a cathode.
However, since nitrogen gas and water produced on the cathode tend to penetrate to the anode through an electrolyte membrane, the concentration of the hydrogen, which is re-circulated and re-used, lowers gradually. Fuel cell's capability to generate electrical power is dependent on the hydrogen concentration in the anode and degrades when the hydrogen concentration lowers. To prevent fuel cell's capability to generate electrical power from degrading, it is necessary to purge the hydrogen from the cathode, which contains impurities, at appropriate timings. It is also necessary to exhaust the purged hydrogen to the atmosphere after making the concentration of the purged hydrogen below a predetermined level (for instance, approximately 4%) by having the purged hydrogen passed through a dilutor.
Accordingly Japanese Laid Open Application No. 2004-281237 discloses a technique in which the hydrogen concentration is measured with a hydrogen concentration meter on the gas discharged to the atmosphere, a purge valve is controlled according to the measured hydrogen concentration and the hydrogen concentration of the gas discharged to the atmosphere is kept below a predetermined level. However there is a problem with using a hydrogen concentration meter because using a hydrogen concentration meter requires an additional space within a fuel cell system and leads to a cost increase of the fuel cell system and a more complicate assembly process as well.
Therefore a fuel cell system without a hydrogen concentration meter has been developed, in which purging is not allowed if a cumulative quantity of the air which is introduced from the outside atmosphere and flows into a dilutor through fuel cell's cathode exceeds a predetermined air quantity. According to this newly developed fuel cell system, purging is to be automatically done without the hydrogen concentration meter if the hydrogen concentration lowers below a certain level.
However in the case of the fuel cell system without a hydrogen concentration meter, if there is no difference in the cumulative quantity of the air flowing into the dilutor, the decrease in the hydrogen concentration in the dilutor differs between various air flow rates (quantity of air flowing for a unit time). Generally speaking, if the cumulative quantities of the air flowing into the dilutor are identical, the smaller the flowing rate, the quicker the hydrogen concentration decreases.
Hence the predetermined air quantity is set in accordance with a relatively large flow rate of air. However in this case no hydrogen purging is allowed until the quantity of the air flowing into a dilutor reaches a predetermined air quantity and there is a problem that purging can not be efficiently done.
This invention has been completed taking account of the problems above mentioned. Making use of a fuel cell system and a control method of the fuel cell system both of which this invention provides, the fuel gas is to be efficiently purged without using a hydrogen concentration meter and the power generation is to be stabilized.